Gate and other sliding stem valves used in fluid flow control are opened and closed by inserting an obstruction or fluid control device into the flow. These valves are operated by a valve actuator employing a variety of methods, including manual actuation, diaphragm-type actuation, and hydraulic actuation. The actuator is usually mounted to the valve and provides the reciprocating linear motion required to open, close, and position the flow control device within the valve.
In the case of a hydraulic actuator, a hydraulic control feed is used to actuate the valve. A control feed line provides hydraulic pressure which actuates a piston in the actuator. As the piston moves, it opens and closes the valve. Generally, opening or closing the valve requires a high control feed pressure. When the desired degree of open or closed is reached, the control line can be locked-in to maintain the pressure. When there is a sudden drop in control feed line pressure, such as due to a failure in the feed line, the valve position is no longer locked-in. Depending on the configuration of the valve and the pressure in the flow line, the valve will tend to fail either in a fully open or a fully closed position. A pressure differential across the flow control device causes an ejection force tending to move the flow control device upward. Most prior art actuators compensate for this by forcing the flow control device either to it upper most position or down to its lower most position, thus fully opening or closing the valve. However, in some situations it is desirable for the valve to resist the ejection force and remain at the degree of open or closed it had been before the loss of control feed pressure.
It is therefore desirable to provide a hydraulic actuator that fails as-is, thus remaining in a given position in the event of a drop in control line pressure. It is also desirable that the actuator be compact and reliable and resist ejection force.